1. Field of the Invention
This invention relates to a method for the production of a pyromellitic dianhydride which is prevented from being hydrated and more particularly to a method for the production of pyromellitic dianhydride, characterized by the fact that while pyromellitic dianhydride formed by cooling a pyromellitic dianhydride-containing gas is in the process of being collected, pulverized, and stored, the water content in an atmosphere accommodating the produced pyromellitic dianhydride is set at a level of not more than 4 vol. % when the temperature of the atmosphere is not higher than 120xc2x0 C.
2. Description of the Related Art
Pyromellitic dianhydride is a subliming substance and is mainly useful as a raw material for such heat-resisting polymers as polyimide resin or as a hardening agent for epoxy resin. The methods which are available for the production of such pyromellitic dianhydride in high purity come under the two types, namely those of the type resorting to the gas phase oxidation and those of the type resorting to the liquid phase oxidation.
The methods of liquid phase oxidation enjoy the advantage of manifesting a high selectivity and nevertheless suffer from the disadvantage of inferior productivity because they are implemented in a batch process. The methods of liquid phase oxidation produce pyromellitic acid and, therefore, require this product to be converted by the removal of the elements of water and necessitate a drying step at an elevated temperature, a subliming and recrystallizing step, or a step for the removal of the elements of water involving use of a large volume of acetic anhydride. Since these methods possibly give rise to such heavy metals and halogenides as originate in the catalysts used in the relevant reaction solutions, they further necessitate a step for separating and recovering such impurities from the respective products or a step for disposing waste liquid emanating from the plant.
Another known method produces pyromellitic dianhydride by subjecting such a tetraalkylbenzene as 1,2,4,5-tetraethyl benzene or 1,2,4,5-tetramethyl benzene to catalytic gas phase oxidation with a molecular oxygen-containing gas. The method of gas phase oxidation permits continuous production of pyromellitic dianhydride. Particularly, this method forms the pyromellitic dianhydride in the reaction gas owing to the catalytic gas phase oxidation and therefore obviates the necessity for depriving the product of the elements of water. The method of gas phase oxidation, therefore, is at an advantage in directly obtaining pyromellitic dianhydride of high purity by being implemented in combination with gas phase collection utilizing reverse sublimation.
Generally, the methods which are available for recovering in the form of crystals the pyromellitic dianhydride which has been collected by the technique of reverse sublimation are known in the three types, those of the type using the procedure which comprises separating a pyromellitic dianhydride-containing gas in a cooling layer furnished with fine holes and raking the crystals by rotating teeth of the shape of a comb or brush; those of the type using the procedure which comprises introducing a pyromellitic dianhydride-containing gas as accompanied by wear-resistant particles into a cooling device and peeling resultantly separated crystals by collision of the particles; those of the type using the procedure which comprises inflicting a mechanical shock as with an air knocker on a cooling device thereby peeling the crystals from the cooling surface; and those of the type using the procedure which comprises elevating the temperature of the wall face having deposited thereon crystals of pyromellitic dianhydride to a level higher than the subliming temperature of pyromellitic dianhydride under the operating pressure inside a collecting device thereby removing the adhering crystals by sublimation and causing the remainder of grown crystals to detach themselves from the wall face and fall down.
The official gazette of JP-A-10-265,474 discloses a method for recovering pyromellitic dianhydride by introducing a pyromellitic dianhydride-containing gas into a vertical recovering column provided with a crystal deposition surface, causing the gas to deposit pyromellitic dianhydride in the form of crystals on the crystal deposition surface and elevating the temperature of the crystal deposition surface to a level in the range of 210xcx9c260xc2x0 C. thereby causing the crystals to peel and fall from the crystal deposition surface.
After the grown crystals of pyromellitic dianhydride have undergone such steps as peeling, exfoliating, and pulverizing, the practice of recovering the pulverized crystals and stowing the recovered pulverized crystals in such a container as the bag for storage is generally followed. As a technique for producing pyromellitic dianhydride by utilizing reverse sublimation, the official gazette of JP-A-07-278,153 discloses a method which effects collection of pyromellitic dianhydride by subjecting tetraalkylbenzene to catalytic gas phase oxidation and cooling the gas thereby inducing precipitation of crystals of pyromellitic dianhydride. The official gazette discloses a method for the production of pyromellitic dianhydride which comprises a series of such steps as cooling a pyromellitic dianhydride-containing gas obtained by the catalytic gas phase oxidation of tetraalkylbenzene with a heat exchanger to about 25020  C., then transferring the cooled gas to a pre-cooler, cooling it therein to a temperature in the range of 150xcx9c200xc2x0 C., and withdrawing the precipitate through an extracting line and meanwhile cooling and collecting the residual gas in a subsequent main collecting device.
The crystals obtained freshly from the collecting device are too hot to be handled as a finished product. Thus, the practice of recovering such crystals, cooling and pulverizing them, and then packing the cooled and pulverized crystals for shipment is generally followed. In the case of obtaining a pyromellitic dianhydride-containing gas by the reaction of catalytic gas phase oxidation and cooling and collecting the crystals of pyromellitic dianhydride, the reaction gas contains the water by-produced by the reaction of catalytic gas phase oxidation. In concert with this cooling of the reaction gas, the water reacts with pyromellitic dianhydride and this reaction induces conversion of pyromellitic dianhydride into pyromellitic monoanhydride (PMMA) and pyromellitic acid (PMA) by hydration. These products produced by the addition of the elements of water are impurities in the target product and form a cause for degrading the purity of the product. For the purpose of preventing the pyromellitic dianhydride from yielding to ring scission, the practice of adjusting the cooling in the main collecting device so as to keep the temperature from falling below 110xc2x0 C. has prevailed heretofore (the official gazette of JP-A-07-278,153). The reason for this adjustment is that since the reaction of catalytic gas phase oxidation suffers the reaction gas to contain a water component without fail, the water component is inevitably condensed and precipitated in consequence of the fall of the temperature to a level below the dew point of water. Since this method prohibits the fall of the temperature below 110xc2x0 C., however, it is deficient in efficiency of cooling and production is substantially in need of a plurality of collecting devices including the main collecting device.
The aim of provisionally hydrating the crystals and then reverting them by the elimination of the elements of water, however, is at a disadvantage in necessitating initial investment, requiring incorporation of a separate step, and raising the price of the product.
The present inventors made an elaborate study about the relation between the ratio of the conversion of pyromellitic dianhydride crystal by hydration and the water content of the atmosphere. They have consequently found that the amount of pyromellitic acid (PMA) to be formed abruptly increases when the water content in the atmosphere exceeds 4 vol. %, whereas the hydration of the pyromellitic dianhydride can be prevented even under the temperature condition of not higher than 120xc2x0 C. when the water content in the atmosphere is not more than 4 vol. %. This invention has been perfected as a result.
According to this invention, pyromellitic dianhydride can be produced while preventing it from being hydrated. In this case, the hydration can be prevented efficiently by introducing an inert gas having a water content of not more than 4 vol. % into the atmosphere enveloping the crystals of pyromellitic dianhydride till the atmosphere is displaced with the inert gas.